A combustible gas monitor is the one instrument in a plant that is judged entirely on the day it matters. The rest of the time it produces a flat line, and that flat line is easy to trust for the wrong reasons: a sensor whose sensitivity has quietly halved shows the same reassuring zero as a healthy one. This article is about the parts of LEL monitoring that decide whether the flat line means anything – where the alarm thresholds come from, where sensors belong, what calibration proves, and what a single-gas calibration does to readings in a mixed atmosphere. Units of measurement and the choice between sensing technologies are covered separately in Methane Detection Explained: LEL, %vol and ppm Ranges and NDIR vs Catalytic Bead Sensor for LEL Methane; everything below assumes those two decisions have been made.
What LEL monitoring measures, and why the scale is relative
LEL monitoring tracks how close an atmosphere is to being flammable. The lower explosive limit is the concentration below which a gas-air mixture cannot sustain combustion, and the %LEL scale expresses concentration as a fraction of that limit rather than as an absolute amount. For methane the limit is 5% by volume in air, so 100% LEL equals 5% vol and 1% LEL equals 500 ppm. The consequence is that the same reading means different absolute concentrations for different gases, and that a %LEL instrument tells you about explosion risk and nothing about exposure or process content.
This is also why the scale ends where it matters least. Above 100% LEL the mixture is flammable and the number has done its job; above the upper explosive limit it becomes too rich to burn, which is a hazard a %LEL channel cannot describe at all. Plants that need to see that region – biogas, landfill collection, purge verification – need a percent-by-volume channel alongside LEL monitoring, not a re-scaled version of it.
LEL monitoring alarm levels: where 10% and 20% LEL come from
Two thresholds do almost all the work in industrial practice, and both are set far below the flammable point on purpose:
- First alarm at 10% LEL – a warning that a release has started. At this level the atmosphere is one tenth of the way to flammable, the leak is usually small, and the correct response is investigation rather than evacuation.
- Second alarm at 20% LEL – an action level. Work stops, ignition sources are controlled and people leave the area while the source is isolated. Higher set points, typically 40% to 60% LEL, are used for automatic shutdown and ventilation interlocks in unmanned installations.
The margin between 20% LEL and a flammable mixture is not generosity, it is compensation for everything the instrument cannot see: the sensor measures at one point while gas moves in a plume, response is not instantaneous, and calibration accuracy is finite. A monitoring plan that sets the first alarm at 50% LEL to reduce nuisance events has not reduced nuisance events; it has spent the entire safety margin and left nothing for the day the release is real.
Fixed and portable LEL monitoring answer different questions
Fixed LEL monitoring watches a place. Sensors sit where gas is expected to accumulate, they run continuously, and their value is that they see a release when nobody is present. Portable LEL monitoring watches a person or a task: a personal instrument confirms the air where the worker actually is, and a sniffer-type instrument searches for the source once a fixed system has raised an alarm. Neither substitutes for the other, and the most common design mistake is buying one and reporting the coverage of the other.
The two also impose different engineering constraints on the sensor. A fixed head is mains-powered and lives outdoors for a decade, so mechanical robustness and temperature range dominate. A portable or wireless unit lives on a battery, so average power consumption dominates: this is the reason the same gas and the same range can lead to two different sensor choices, and why the power column in the comparison below is the one that changes designs.
Sensor placement in LEL monitoring: density decides the height
Gas does not distribute itself evenly to make monitoring convenient. Methane is lighter than air and collects at high points, under roof structures and in the top of enclosures. Propane, butane and heavier hydrocarbon vapours are denser than air and pool at floor level, in pits, trenches and sumps. A sensor mounted at breathing height in a propane facility can read zero while a flammable layer builds up a metre below it, and the same sensor in a methane installation can read zero while gas accumulates above it.
Two further factors are routinely missed. Ventilation moves the plume: a sensor placed downstream of a fan may see a release seconds after a sensor placed near the likely source, and mechanical ventilation can dilute a real leak below the first alarm level entirely. And enclosures create their own geometry – inside a cabinet or a skid, gas follows the internal volume, not the room. Placement should follow a written assessment of release points, gas density and airflow, and that assessment should be revisited whenever process equipment moves.
Calibration and bump testing in LEL monitoring: what each one proves
Calibration and a bump test answer different questions, and confusing them is how monitoring systems fail quietly. A calibration applies a known concentration of the target gas and adjusts the instrument so its reading matches: it establishes accuracy across the range. A bump test – sometimes called a functional test – applies gas briefly and confirms only that the sensor responds and the alarm activates. A bump test cannot tell you that the sensor is accurate; a calibration cannot tell you that it is still working tomorrow.
The practical routine, in the order it should appear in a procedure: first, calibrate on schedule with certified gas and record the as-found reading before adjustment. Second, bump test frequently – for portable instruments before each use – since most field failures are blocked inlets, expired sensors and dead batteries rather than gradual drift. Third, treat every calibration failure as data: a sensor that needed a large correction twice in a row is on its way out, whatever the current reading says. And last, log the target gas: an instrument calibrated on methane and used in propane service is not out of calibration, it is measuring something else, which brings us to cross-sensitivity.
Cross-sensitivity in LEL monitoring: one calibration, many gases
A combustible gas sensor is calibrated on one gas and used in an atmosphere that may contain several. The reading it produces for a different hydrocarbon is related to the calibration gas by a response factor, and those factors are not close to one. This has two practical consequences. In a mixed atmosphere a %LEL reading is a composite, not a specific measurement, so an instrument reporting 15% LEL is telling you about total flammability rather than about a named gas. And in single-gas service the calibration gas must match the hazard: a monitor calibrated on methane and deployed to guard a propane tank farm will misreport the very release it was installed for.
Infrared sensors add a second dimension to the same issue. NDIR detects only molecules with an absorption band at the chosen wavelength, so a hydrocarbon channel is blind to hydrogen and acetylene, which a catalytic bead sensor would see. That is a legitimate design trade-off when the hazard is hydrocarbons and the instrument runs on a battery, and it is a gap that must be filled by a second sensing principle when hydrogen is credible. Ask for the cross-sensitivity data as a document, at the design stage – if a supplier cannot produce it, the specific-gas claim in the datasheet is unverified.
Certification for LEL monitoring in Class I, Div 1 and Zone 0
In hazardous locations the sensor marking decides where the finished instrument may legally be installed, and North American and international schemes describe the same physics in different words. The MIPEX sensors used for LEL monitoring carry both at once. MIPEX-05 holds an ATEX EU-Type Examination Certificate UL 25 ATEX 3487U Rev. 0 and an IECEx Ex Component Certificate IECEx UL 25.0093U, marked I M1 / II 1G Ex ia op is I Ma / Ex ia op is IIC Ga, with the product label also showing Class I, Zone 0, AEx ia op is IIC Ga and UL certification issued for both the United States and Canada. MIPEX-02 holds UL 23 ATEX 3072U Rev. 2 and is marked Ex ia IIC Ga to UL 913 and CAN/CSA-C22.2 No. 157-92, again with Class I, Zone 0 marking.
Two details in those strings decide project timelines. The letters «op is» mean the optical radiation itself has been assessed as inherently safe, which is what permits an infrared source inside a Zone 0 instrument. The letter «U» at the end of a certificate number means the sensor is approved as an Ex component and not as standalone apparatus: the instrument built around it is certified separately by its manufacturer using the component parameters from the sensor documentation. No sensor certificate covers a finished detector, and a supplier who implies otherwise has not read their own paperwork.
Choosing a sensor for LEL monitoring: NDIR options compared
All three sensors below are intrinsically safe at level «ia» and use the same dual-wavelength infrared method, so the selection is driven by power budget, housing and the type of installation rather than by the measuring principle.
| Parameter | MIPEX-02 | MIPEX-04 | MIPEX-05 |
| Average power consumption | ≤ 2.5 mW | around 0.1 mW | ≤ 0.2 mW |
| Intended installation | fixed heads, harsh outdoor environments | portable individual detectors and wireless monitoring systems | portable, wireless and battery-powered instruments |
| Housing | plastic or stainless steel, Ø 20.1 × 16.6 mm – standard 4th series size | 52 × 24 × 18 mm | plastic Ø 20.1 × 16.6 mm – standard 4th series size |
| Approvals | IECEx, ATEX, UL | IECEx, ATEX, UL | IECEx, ATEX, UL |
| Notable property | metal housing option, 4th series size | record-low power consumption | 4th series size and ultra-low power consumption |
| High-sensitivity variant | – | MIPEX-04 ppm, 10 ppm resolution | MIPEX-05 ppm, 10 ppm resolution |
A dash means the figure is not stated in that model’s published documentation; we confirm such values on request rather than estimate them here. The power column is what changes designs: moving from 2.5 mW to 0.2 mW is the difference between an instrument serviced every season and one that runs for years on the same cell.
LEL monitoring: questions engineers ask
What does 20% LEL actually mean?
It means the atmosphere contains one fifth of the concentration needed to sustain combustion. For methane, whose lower explosive limit is 5% by volume, 20% LEL corresponds to 1% by volume in air. It is an action level in most industrial procedures: work stops and the area is cleared while the source is found.
How often should an LEL monitor be calibrated?
Calibration follows the manufacturer’s interval and the site’s risk assessment, and the as-found reading is recorded before any adjustment so drift is visible over time. Between calibrations, a bump test confirms that the sensor still responds and the alarm still activates – for portable instruments, before each use.
Can one LEL sensor cover methane and propane?
An infrared hydrocarbon sensor responds to both, but with different response factors, so the reading is accurate only for the gas it was calibrated on and approximate for the others. Where a specific reading is required for each gas, either calibrate for the dominant hazard or use separate channels.
Why does a %LEL instrument read zero in a very rich atmosphere?
Above the upper explosive limit some sensing technologies saturate or lose signal, and catalytic types can be damaged by prolonged over-range exposure. Percent-by-volume measurement, not a %LEL channel, is the right tool above the flammable range.
Does a sensor certificate make our finished detector certified?
No. A certificate number ending in «U» is a component approval: it lets a manufacturer build the sensor into an instrument, which is then certified separately against the component parameters. Ask for both documents – the sensor’s component certificate and the instrument’s own approval.